It could be shared with people you don’t intend to share it with. The person you are sexting might forward it to just one or two friends, who forward it to a few more, and so on. Or somebody else using that phone could stumble across it by accident. Or the recipient could decide to spread it around if they get mad at you.
Adults could see it. A sext message that gets loose can be seen by your parents, teachers, coaches, or the people you babysit for. How mortifying
It could backfire. You might send a sext intending to impress or interest somebody, but it could turn them off instead. They might get the wrong impression of you – the exact opposite of what you wanted.